This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The mechanism of dinitrogen reduction at the iron-molybdenum cofactor (FeMoco) of nitrogenase is one of the great mysteries of bioinorganic chemistry, and recent evidence suggests that substrates bind at iron. However, nitrogenase has many iron atoms, making it difficult to analyze spectroscopically. There is an urgent need for one- and two-iron complexes with the same spin state and geometry as the iron atoms in nitrogenase, to use as spectroscopic models for interpretation of the data on nitrogenase. Our hypothesis is that reductive activation of the FeMoco generates transient unsaturated iron sites for substrate binding and reduction. These iron binding sites would have highspin iron in a ligand-poor environment, with a geometry quite different from that in most known iron complexes but similar to the FeMoco iron sites. Our preliminary evidence shows that the geometry of an iron complex has a dominant influence on its orbital energies, which in turn determine the strength of binding N2 and other small molecules. Therefore, the proposed research focuses on compounds in which the iron atom has similar geometry (trigonal pyramidal), electronic structure (high-spin with weak-field ligands) and spectroscopy as the FeMoco iron sites. The unusual coordination environment will be enforced by coordinating iron to mutants of the azurin apo-protein, which is known to constrain metals to a trigonal pyramidal geometry. The iron complexes will be evaluated by ENDOR, infrared, Raman, M[unreadable]ssbauer, and x-ray absorption. The spectroscopic results will provide a link between the structures of novel model compounds and the known data for nitrogenase, and the x-ray absorption experiments will show the oxidation level,geometry, and binding modes to the iron atom. Preliminary results from NMR indicate binding of azide to the iron atom in some engineered azurin-iron complexes, and we will use x-ray absorption to analyze this and other novel iron species.